Hcl Strong Or Weak Acid

Strong Acids and Weak Acids

Classification Scheme based on pK_a

The dissociation of an acid HA is determined by its acidity constant 1000_a:

(1)

HA = H⁺ + A^-

with Yard_a = [H⁺][A^-] / [HA]

Potent acids dissociate completely in water, while weak acids do not dissociate completely. A classification based on acerbity constants or pK_a values seems natural.

Let's denote the total corporeality of the acid past C_T ≡ [HA]_T (which is de facto the acrid's initial concentration earlier it dissolves). In the equilibrium state, the total concentration splits into its undissociated and dissociated parts:

Strong and weak acids then differ equally follows (greatly simplified):

	Strong Acrid	Weak Acrid
acidity constant:	1000_a ≫ ane	K_a ≤ 1
pK_a = -log Thousand_a	pK_a < 0	pK_a > 0
[H⁺] = 10^-pH	[H⁺] ≈ C_T	[H⁺] ≪ C_T
undissociated acid:	[HA] ≈ 0	[HA] ≈ C_T
dissociated acid:	[A^-] ≈ C_T	[A^-] ≪ C_T

In literature, there is no sharp border line betwixt what we call a strong acid and what we call a weak acid. More refined classification schemes distinguish fifty-fifty betwixt very strong acids, stiff acids, weak acids, very weak acids etc. Concerning aqion, however, we prefer the simple sectionalization into two groups:

	strong acids:	acids with pK_a < 0
	weak acids:	acids with pK_a > 0

Polyprotic Acids. The idea remains valid even for N-protic acids, H_NA. The acidity constant 1000_a should only be replaced past the 1^st dissociation constant M₁. The mathematical description is straightforward (run across Appendix):

(3)

undissociated fraction = $\dfrac{ane}{ane+K_1/x}$

with x = [H⁺] = x^-pH

The diagram below displays the undissociated fraction of some common acids (based on 3). The minor circles marking the respective pK₁ values. As expected, stiff acids are completely dissolved in real-globe applications (pH > 0).

$undissociated fraction of strong and weak acids$

Group 1: Strong Acids with pK_a < 0

Potent acids used in aqion are:

hydroiodic acrid	Howdy	pK_a = -x
hydrobromic acid	HBr	pK_a = -9
hydrochloric acrid	HCl	pK_a = -vi
sulfuric acrid (1^st dissociation step)	H₂SO₄	pK_a = -3
selenic acid (1^st dissociation footstep)	H₂SeO₄	pK_a = -three
nitric acid	HNO₃	pK_a = -1.32
chromic acid	H₂CrO₄	pK_a = -0.86

Notation: Due to the fact that Howdy, HBr, HCl, H₂SO₄, H₂SeO₄, and HNO_iii near do not exist in undissociated form their first dissociation step is not explicitly contained in the thermodynamic database.^one

Example calculations with stiff acids are presented here.

Group 2: Acids with pK_a > 0 (Weak Acids)

In dissimilarity to the strong acids with negative pK_a values, acids with pK_a > 0 are explicitly divers in the thermodynamic database of aqion by their log One thousand values. Here are some examples listed in the gild of decreasing strength (valid for standard weather at 25 and one atm):

Reaction Formula	log Grand	pK	Ref.
HSeO₄ ^- = H⁺ + SeO_iv ^-2	-one.66	i.66	[W]
HSO₄ ^- = H⁺ + Then₄ ^-two	-1.988	1.988	[W]
H_iiiPO₄ = H⁺ + H_twoPO₄ ^-	-two.147	ii.147 ²	[M]
Fe⁺³ + H₂O = H⁺ + FeOH⁺²	-two.xix	2.xix	[W]
H₃AsO_iv = H⁺ + H₂AsO_four ^-	-2.3	ii.3	[West]
H₃Citrate = H⁺ + H₂Citrate^-	-3.128	3.128	[M]
H_twoSeO₃ = H⁺ + HSeO_iii ^-	-3	3	[Westward]
HF = H⁺ + F^-	-3.18	3.eighteen	[West]
HNO_ii = H⁺ + NO₂ ^-	-3.22	iii.22	[E,L]
HFormate = H⁺ + Formate^-	-iii.753	three.753	[M]
H₂Se = H⁺ + HSe^-	-3.8	3.8	[Due west]
HLactate = H⁺ + Lactate^-	-three.863	3.863	[Due east,L]
H₂MoO_iv = H⁺ + HMoO_four ^-	-3.865	three.865	[M]
HMoO₄ ^- = H⁺ + MoO_iv ^-two	-4.290	4.290	[M]
HAcetate = H⁺ + Acetate^-	-4.757	4.757	[M]
H₂Citrate^- = H⁺ + HCitrate^-2	-4.761	4.761	[K]
Al⁺ⁱⁱⁱ + H₂O = H⁺ + AlOH⁺ⁱⁱ	-5.0	5.0	[W]
H₂CO_three ^* = H⁺ + HCO_iii ^-	-six.351	six.351 ³	[Due west]
HCitrate^-two = H⁺ + Citrate^-3	-6.396	vi.396	[G]
HCrO_iv ^- = H⁺ + CrO_four ^-two	-6.509	vi.509	[Thousand]
H_iiS = H⁺ + HS^-	-half dozen.994	6.994	[Westward]
H₂AsO_iv ^- = H⁺ + HAsO_four ^-2	-7.16	vii.16	[W]
H_twoPO_four ^- = H⁺ + HPO₄ ^-two	-7.207	7.207	[W]
HSeO₃ ^- = H⁺ + SeO_iii ^-two	-8.5	viii.v	[W]
H₃AsO_iii = H⁺ + H₂AsO₃ ^-	-9.15	9.15	[W]
H₃BO₃ = H⁺ + H₂BO₃ ^-	-9.24	nine.24	[W]
NH_four ⁺ = H⁺ + NH₃	-9.252	9.252	[W]
H_ivSiO₄ = H⁺ + H₃SiO₄ ^-	-9.83	9.83	[Due west]
HCO_three ^- = H⁺ + CO_three ^-2	-10.329	x.329	[Due west]
HAsO₄ ^-2 = H⁺ + AsO₄ ^-3	-xi.65	11.65	[W]
HPO₄ ^-2 = H⁺ + PO_iv ^-iii	-12.346	12.346	[W]
HS^- = H⁺ + Due south^-ii	-12.918	12.918	[W]
H₃SiO₄ ^- = H⁺ + H_iiSiO_four ^-two	-13.17	xiii.17	[W]

As mentioned above, the strong acids with pK_a < 0 (e.g. Hullo, HBr, HCl, H_iiAnd so_four, HNO_iii) are not present in this tabular array (and database). In addition, acidity constants of many organic acids are presented here.

These acids are bachelor equally inorganic and organic reactants in the reaction tool (pH calculator). Calculated pH values for i, 10, and 100 mM are shown in this table.

Weak Acids vs Dilute Acids

A weak acrid and a dilute acrid are two different things, like apples and oranges. The get-go relies on the acidity constants Chiliad_a (which is a thermodynamic holding of the acid that no one can change) while the 2d relies on the amount C_T of a given acid:

	weak acid	↔	strong acid	⇔		small M_a	↔	large Yard_a
	dilute acid	↔	concentrated acid	⇔		small C_T	↔	large C_T

Y'all cannot make a weak acid stiff, simply you tin can change the degree of dilution (or concentration) as you like. The principal differences between the caste of strength and the degree of dilution can be summarized every bit follows:

	degree of Forcefulness	caste of Dilution
determined by:	acerbity constant Yard_a	amount of acid C_T
relationships:	weak acid ↔ strong acid	dilute acid ↔ concentrated acid
	small K_a ↔ large K_a	small C_T ↔ large C_T
	(positive pK_a ↔ negative pK_a)
compares:	ii unlike acids	dilution of the aforementioned acid
describes:	release of H⁺	dilution of H⁺
type:	fundamental property	control parameter
	(cannot be changed)	(tin can exist changed)

Instead of Chiliad, the classification can as well be based on pK, as indicated by the pK-C_T diagram beneath. (Annotation: For polyprotic acids pK refers to the first dissociation step.)

weak/strong vs dilute/concentrated acids

Appendix – Undissociated Fraction of Acrid

Given is an Northward-protic acid H_NA characterized past Northward acidity constants K₁ to K_N. The sum over all acid species defines the full concentration (mass remainder):

(A1)

C_T ≡ [H_NA]_T = [H_NA] + [H_N-1A^-] + … + [A^-N]

The fraction of the undissociated species is expressed by the ionization fraction a₀:

(A2)

undissociated fraction:

a₀ = [H_NA] / C_T

Its pH dependence is given by:^iv

(A3)

$a_0(x) \,=\, \left( 1+\dfrac{K_1}{ten} + \dfrac{K_1K_2}{x^ii}+ \dfrac{K_1\cdots K_N}{ten^Due north} \right)^{-one} \approx\, \left( 1+\dfrac{K_1}{x} \correct)^{-1}$

where x is an abbreviation for [H⁺] = 10^-pH.

References

[Eastward]	Database EQ3/6 taken from: T.J. Wolery: EQ3/six, A Software Package for Geochemical Modeling of Aqueous Systems: Package Overview and Installation Guide (Version 7.0), Lawrence Livermore National Laboratory UCRL-MA-110662 PT I, Sep 1992
[L]	Database llnl taken from: 'thermo.com.V8.R6.230' prepared by Jim Johnson at Lawrence Livermore National Laboratory, in Geochemist'south Workbench format. Converted to PhreeqC format by Greg Anderson with help from David Parkhurst (llnl.dat 4023 2010-02-09 21:02:42Z dlpark)
[M]	Database minteq taken from: J.D. Allison, D.S. Brown, K.J. Novo-Gradac: MINTEQA2/PRODEFA2, A Geochemical Cess Model for Environmental Systems, Version 3.0, User's Manual, EPA/600/3-91/021, March 1991
[W]	Database wateq4f taken from: J.W. Ball and D.K. Nordstrom: WATEQ4F – User's manual with revised thermodynamic information base and exam cases for calculating speciation of major, trace and redox elements in natural waters, U.S.G.S. Open up-File Report 90-129, 1991